A hydrogenated amorphous silicon thin film (hereinafter referred to as a-Si:H thin film) has been put to practical use as a pixel switching transistor for liquid crystal displays, as an optical sensor serving as an image sensor for facsimile machines, as a solar cell used as a battery for calculators, and the like. The biggest advantage of this a-Si:H thin film is that the film can be fabricated stably on a large-area substrate with good reproducibility at a process temperature of only about 300° C. However, with an increase in pixel density in liquid crystal displays and image sensors, silicon semiconductor thin films compatible with a faster drive have been demanded. Conventional transistors using an a-Si:H thin film had a mobility of 1.0 cm2/V·sec at most, and thus did not have sufficient performance to satisfy the demand. Thus, in order to improve mobility, development of techniques for crystallizing a-Si:H thin films has been pursued. Methods for the crystallization include, for example, the following techniques.
1) Depositing a thin film on a substrate by plasma enhanced CVD, using a source gas in which hydrogen or SiF4 is mixed in silane gas, and then crystallizing the thin film.
2) Using an a-Si thin film as a precursor, attempting to crystallize the a-Si thin film.
Of these techniques, the crystallization method described in the technique 2) includes, for example, a solid phase growth technique, in which heat treatment is performed at about 600° C. for an extended period of time, and an excimer laser annealing technique.
With, in particular, the latter excimer laser annealing technique, a polycrystalline silicon thin film with high mobility (>100 cm2/W·sec) has been successfully obtained without the need to actively increase the substrate temperature. This fact is described in detail, for example, in IEEE Electron Device Letters, 7 (1986), pp. 276-278 and IEEE Transactions on Electron Devices, 42 (1995), pp. 251-257.
When a TFT of the above-described a-Si:H thin film or polycrystalline silicon thin film is used, as the switching transistor, in the pixel portion of a liquid crystal display, an ON current is required which is sufficient to write signals applied to the TFT to the liquid crystal (layer) within a given time period, and in addition a reduction of leakage current in an OFF state is required. Further, in a built-in type liquid crystal display having a drive circuit provided on the periphery of the substrate, when a TFT of the polycrystalline silicon thin film is used in the drive circuit, the performance and reliability of each TFT need to be sufficiently assured as a circuit element.
In order to satisfy these requirements, for example, in a TFT having an a-Si:H thin film, the source region and the drain region are doped with impurities so as to reduce leakage current. In addition, in a TFT having a polycrystalline silicon thin film, an offset structure or an LDD structure is employed so as to maintain the performance and reliability of the TFT, and at the same time, to reduce so-called leakage current in an OFF state. (Note that the term “offset structure” refers to a structure in which appropriate space (for example, 0.5 μm) is provided between the channel portion (which is located immediately below the gate electrode in the case of a top-gate type TFT) and each of the source and drain regions of the semiconductor, and that the term “LDD structure” refers to a structure in which between the channel portion (which is located immediately below the gate electrode) and each of the source and drain regions of the semiconductor a doping region is provided in which impurities with a lower concentration than those of the source and drain regions are diffused.)
Future demands in, for example, liquid crystal displays would be for low cost and image quality (for example, a display grade having a resolution such as that of photo image quality), and the like. For satisfying such demands, very fine pixels and fast operation of an in-built drive circuit are, of course, required in liquid crystal displays, and technically, fabrication of an extremely small TFT becomes an important and essential technique.
If an extremely small TFT is realized, it is possible in, for example, a TFT used in the pixel portion (hereinafter referred to as TFT for a pixel) to further increase the aperture ratio of the pixel, reduce the capacity level of parasitic capacity, improve image quality, and increase driving speed. In addition, in a TFT used in an in-built drive circuit (hereinafter referred to as TFT for a drive circuit), the capacity level of parasitic capacity is reduced, achieving an even faster drive.
Note, however, that fabrication of an extremely small TFT is accompanied by other problems to overcome. One of the problems in view of the TFT for a pixel is that it is necessary to further reduce a leakage current in an OFF state of about 10−12 A, which is conventionally obtained, by one digit or more to reduce the brightness difference in the plane of the panel. If this problem is not overcome, even if an extremely small TFT reduces an area per pixel and a storage capacity portion for storing an electric charge of signals, it becomes difficult to realize a bright display without causing a reduction in aperture ratio. In addition, as for the foregoing problems in view of the TFT for a drive circuit, in employing the above-described offset structure or LDD structure, factors in fabrication such as fine processing accuracy and alignment accuracy for photolithography become critical limitations. Further, since the offset structure and the LDD structure require stable characteristics and also require the structure to be self-aligned, the fabrication process becomes more complicated, causing an increase in costs.
Moreover, a TFT for a pixel or a TFT for a drive circuit used in liquid crystal displays, a display and image-input integrated panel, an optical sensor serving as an image sensor used in facsimile machines, a solar cell used as a battery for calculators, or the like is expected to be developed, with use of a flexible substrate (made of plastic or the like), to a ultra-thin flexible input and output panel capable of connecting to an electronic paper and a network (Internet). Thus, such a flexible substrate also requires techniques for fabricating a thin film transistor, an optical sensor, a solar cell, and the like with excellent characteristics at low cost.
However, provision of TFTs and the like on the flexible substrate requires techniques for fabricating extremely small TFTs on the flexible substrate and improvement in reliability. In addition, because the flexible substrate is inferior in heat resistance to, for example, a glass substrate, the fabrication process temperature needs to be reduced. Moreover, in order to cut fabrication cost, the number of fabrication steps needs to be reduced.
To summarize the above, conventional TFTs have problems described below.
(i) A complicated fabrication process and an increase in costs resulting from the fabrication of extremely small TFTs.
(ii) A reduction in reliability of TFTs resulting from the fabrication of extremely small TFTs.
(iii) A high process temperature upon formation of TFTs on a flexible substrate or the like.